1. Field of the Invention
The present invention relates to a thin film transistor and method for fabricating the same. More particularly, the present invention relates to a thin film transistor and method for fabricating the same wherein a polysilicon layer is used as a semiconductor layer.
2. Description of the Related Art
Generally, a thin film transistor comprises a semiconductor layer, a gate insulating layer and a gate electrode. Further, the semiconductor layer is typically a polysilicon layer whose electron mobility is 100 times higher than that of an amorphous silicon layer.
A polysilicon semiconductor layer is typically manufactured by forming an amorphous silicon layer on the substrate and then crystallizing the amorphous silicon layer. The crystallization method may, for example, include an excimer laser annealilng (ELA) method, a sequential lateral solidification (SLS) method, a method induced crystallization (MIC) method, or a metal induced lateral crystallization (MILC) method.
Among them, the MILC method has the advantages that uniformity properties and reduction of production costs can be obtained more easily than with the ELA method. Moreover, contamination of a channel region due to metal can be reduced compared with the MIC method.
MILC may be performed by coating some regions of the amorphous silicon layer formed on the substrate with a crystallization-inducing metal layer and by performing heat treatment. In the heat treatment, a region that contacts the crystallization-inducing metal layer may be crystallized by the MIC method, and a region that does not contact the crystallization-inducing metal layer may be crystallized by the MILC method.
Crystalline grains formed by the MILC method have an orientation, and are generally grown in an elongated form. Amongst the crystalline grains, a region can exist in which crystallization is not made by the MILC method, and amorphous silicon remains. The region where the amorphous silicon remains can be a factor that degrades electrical characteristics such as electron mobility. It can also decrease the uniformity of the electrical characteristics in the substrate.